In the past few years there has been an increased interest in using plane-parallel flakes as pigments in paints and printing inks, as a catalyst material, as a starting product for magnetic and electric screens, and as a starting material for conductive lacquers. In contrast with the classical pigments produced in accordance with a grinding process and having a more or less spherical shape, plane-parallel flakes are characterized by an improved brilliance and by the fact that their quantity required in a paint as a pigment is substantially smaller. Thus, for example, aluminum flakes produced in accordance with a physical vapor deposition (PVD) process typically have a thickness of 30-500 nm; their superficial dimensions ranging between 5 and 50 microns. Even 3-4 layers of such aluminum flakes produce an optically opaque layer at quantities of no more than 0.3 to 0.4 g/m.sup.2.
Plane-parallel flakes have heretofore been produced, in accordance with the prior art, through a costly, intermittent three-stage process. The high costs of this process and the low production quantities do not allow for use in mass-produced articles such as in metallic paintcoats or printer's inks.
One example for production according to an intermittent. multi-stage process is the production of optically variable pigment flakes used to enhanced safety against forgery oil banknotes such as disclosed in EP 227423. Similar products and processes are disclosed in U.S. Pat. No. 5,278,590.
A process disclosed in U.S. Pat. No. 4,434,010 does not involve use of separating agents. Vapor deposition is instead performed directly onto a plastic film which is subsequently comminuted and dissolved completely in a suitable solvent. As a result, the vapor deposition layer dissociated into particles remains as a suspension.
In all of the above-represented cases, large amounts of solvents are required for washing out the product. These must either be reprocessed or disposed of.
In summary, it may therefore be noted with respect to the prior art that in all of the known PVD processes for producing plane-parallel flakes, with the exception of U.S. Pat. No. 4,434,010, the following stages are passed through:
(1) application of a separating lacquer on a polyester film substrate by printing and drying, with this process being performed on classical rotary gravure printing machines for coiled material in an atmosphere under explosion protection conditions; PA1 (2) sequential vapor deposition of the layers under high vacuum, partly through several reciprocating passages of the film substrate in the vacuum unit, and removal of the vapor coated roll; and PA1 (3) removing the composite product by dissolving the separating lacquer in a solvent (acetone, ethyl acetate), and filtering out the product from the solvent, for which purpose scraping and brushing means are used in a solvent bath through which the web of film passes, with the product subsequently being filtered out and proceeding towards further processing.
The use of inorganic separating agents vapor deposited in a vacuum is described, for different applications, by Rosenfeld in U.S. Pat. No. 5,156,720. In this process, as well, a film substrate preferably comprised of polyester film is used, with this film substrate being usable only once owing to the high strain caused by the process. As one square meter of aluminum flakes having a thickness of 30 nm and a mass of 0.091 g is produced from one square meter of film having a, weight of 16.8 g by using a separating agent in a quantity of approx. 6 g/m.sup.2 in manufacture, the yield by weight is merely 1/272 of used film and separating lacquer. This method is therefore uneconomical. Ratios are somewhat more favorable when using a vapor deposited separating agent with approx. 0.1 g/m.sup.2, however the ratio still is only 1/180. In each case, this is an intermittent process which must be carried out consecutively on 2 or 3 different machines. The production costs are correspondingly high and hitherto thwarted a wider use of the product. Particular vapor deposition or sputtering materials may not even be applied on a film substrate as their thermal resistance is too low.
A vacuum coating machine for the production of oxide layers, which are preferably made of silicon monoxide, on temperature-sensitive plastic films, such as polyethylene and polypropylene, is disclosed in DE 4221800 A1. The purpose of this machine is to increase the barrier properties of the base film for oxygen, water vapor, and aroma substances. The characteristics of web path, evaporation zone, and array of thermal treatment steps allow for the production of oxide layers which are substantially free of micro cracks. This machine allows for the continuous removal of excess condensate, which allows the coating process to operate without the need to shield the rims of the film against, coatings. However, the removal is accomplished by spalling when the belt passes over a deflection roller, or by scraping or brushing the excess condensate.
A system of a closed loop belt to remove waste metal coatings which deposit right and left of a traveling web, or in areas near a melting process under vacuum, is disclosed in French Patent No. 1,507,784. During a melting process under vacuum, splashes and deposits from metal vapor take place. This system allows for the removal of those deposits before a certain build-up thickness has been reached. However, this system is largely inapplicable to the production of plane-parallel flakes.
In view of the foregoing, it would be desirable to provide a technique for producing plane-parallel flakes which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to provide a technique for producing plane-parallel flakes in an efficient and cost effective manner.